Mechanical, single stage, air compressors are well known in the art which comprise one of several different types, such as piston and cylinder type, centrifugal type, axial-flow type, turbine type and even other types. The simplest and most common type in use is the piston and cylinder type. In this type compressor air, or any gas, is admitted via a valve into the cylinder where a reciprocating piston therein compresses the air or gas within the cylinder and displaces the compressed air to a conduit or reservoir from which it can be taken for use as may be required.
Multiple stage air compressors are also well known in the prior art. Such multiple stage compressors are utilized to compress air and/or other gas to pressures which are higher than can normally be achieved with a single stage compressor. These multiple stage compressors normally comprise a plurality of mechanical single stage compressors of any one type or the other, interconnected in series, wherein the compressed gas is passed from one stage to the next with the pressure thereof being increased at each succeeding stage. In the typical multiple stage compressor of the piston and cylinder type, air or gas, at ambient pressure and temperature, is admitted into the cylinder of the first compressor stage where a first reciprocating piston serves to compress the air therein and displace it to the second stage and so on through all the stages in the system, with each stage further compressing the previously compressed gas until the final desired pressure is achieved.
It is also well known that most multistage compressors normally include a cooling step of the compressed air between at least some of the various compressor stages so that the overall compression may be more isothermal than adiabatic. That is to say, because of the ideal gas law, (PV=nRT), each compression stage of the air will, of course, cause an increase in pressure, P, as intended, and will also cause a directly proportional increase in the air temperature, T.
While this in not normally a problem in a typical single stage compressor, where a defined volume of air is compressed but once, the relatively high air pressures obtained in most multistage compressors can result in the compressed air having excessive and problematical temperatures, but for the intermediate cooling of the compressed air between the various compression stages.
For example, compressed air temperatures in excess of 500.degree. F.(about 260.degree. C.) is not only a hazard to persons therearound, but can cause operating difficulties of various different forms, such as malfunctioning valves and other compressor components. As a result, practically all commercially available multiple stage compressors include an intercooler system of some sort between at least some of the compression stages for the purpose of preventing excessive heating of the compressed gases compressed to such high pressure levels.
It is also well known that water exists as vapor in practically any ambient air to be compressed in a conventional compressor, which is quantified as the relative humidity of the air. The relative humidity of the air, expressed as a percent value, is the ratio of (a) the water vapor actually present in the air, in comparison to (b) the saturation vapor pressure at the temperature in question. The saturation vapor pressure is a function of the air temperature, so that as the temperature increases for any given sample of air, the saturation vapor pressure increases, and accordingly, the relative humidity decreases.
In a compressor, the above natural conditions can create a problem. Obviously, when the air is compressed, with little or no externally caused change in temperature, the temperature of the compressed air is increased in proportion to the increase in pressure, as noted above. Because the saturation vapor pressure of water is dependent on the temperature of the air, it follows that when the temperature is increased the saturation vapor pressure is also increased.
Thereafter, if the compressed air is cooled by any means, such as an intercooler, for example, it is not uncommon for the water vapor pressure in the twice-compressed air to actually exceed the saturation vapor pressure for the compressed air. This is, particularly, the case if the compressed air is allowed to further cool thereafter. Therefore, it is not uncommon for this phenomenon to cause significant amounts of water to be condensed as liquid within the system.
Free water within the compressor, however, is known to cause a variety of problems, such as oxidation (rusting) of compressor components, and more importantly, cause condensed water to be admixed into the lubricating oil within the compressor sump. Such dilution of the lubricating oil in the compressor with water can seriously impair the normal operation of the compressor as well as reduce its overall useful life. Therefore, it is highly desirable to eliminate, or to at least substantially minimize, the condensation of such water within any compressor, particularly any such water that may find its way into the lubricating oil.